1. Field of the Invention
This invention relates to computer peripheral data storage devices, and in particular to rotating disk data storage units, such as magnetic disk drives, having a novel servo-independent format.
2. Description of Related Art
Rotating disk data storage units ("disk drives") are commonly used to store data in computer systems. Conventionally, each disk drive comprises one or more rotating disks coated with or made from magnetic or optical recording material. Generally, each disk surface is formatted into a plurality of tracks, each track comprising a plurality of physical sectors. Movable read/write transducer heads positioned proximate to each disk surface are used to record the track and sector formats, and to read data from and write data in each physical sector.
Extensive research efforts in the field of magnetic hard disk drives for many years have been directed to developing practical techniques for increasing areal recording density. Improved techniques for increasing areal recording density have been an important enabling factor in the trend in this field toward smaller, yet higher capacity, disk drives.
Areal recording density is generally expressed in terms of bits per square inch (or other unit area). Analytically, areal density is the product of the track density (i.e., the number of concentric tracks per inch, or "TPI") on the surface of a disk, and the bit density (i.e., the number of bits per inch, or "BPI") that can be recorded along a particular track.
Prior art disk drives have included various known methods of increasing disk capacity by increasing areal recording density. One such method entails increasing the bit density; this method includes increasing the data channel rate. Conventional disk drives employ a single data channel rate (such as 15 or 20 MHz) for data transmission from read-write circuitry to an active read-write head and ultimately to the recording media on the surface of a disk. This single data channel rate, or constant recording frequency, has increased over time through use of higher performance read-write circuitry and high resolution read-write heads and recording media.
The heads of a disk drive are generally positioned by means of a closed-loop servo system. In some designs, one side of one disk in a stack of disks is encoded with special servo information as a dedicated servo disk. A ganged set of heads is positioned in response to servo feedback information read by the head corresponding to the dedicated servo disk. However, this type of design is commonly used only when a relatively large number of stacked disks are used, since no data can be stored on the dedicated servo disk surface.
With smaller numbers of stacked disks, a more common design is to write servo information in "servo sample wedges" embedded around the surface of each disk. Such embedded servo designs may have, for example, 17 or more servo sample wedges per disk surface, dividing the tracks on each surface into an equal number of sectors of a circle which may be referred to as "data wedges". FIG. 1 is a top view of a prior art embedded servo disk surface showing eight servo sample wedges 2, eight data wedges 4, an inner track 6, and an outer track 8. However, in modern embedded servo disk drives, the number of data wedges on a track may be as high 56 or more.
In conventional disk drives, the disks are rotated at a constant angular velocity. As an active read-write head is displaced radially outward from the innermost track, successive circumferential tracks pass beneath the head at an increasing linear velocity. In constant frequency recording, linear bit density along a track is determined by the maximum number of flux changes per inch ("FCI") that may be recorded on the innermost data track of a disk drive. This number is largely determined by the quality of the read-write electronics and the read-write head/recording media combination, and varies from design to design. However, for a particular design, recorded flux transitions become farther and farther apart as radial displacement increases since the circumferential length of a data track is proportional to the track radius. Accordingly, linear bit density on the outermost tracks is substantially less than on the innermost tracks.
This is shown in a stylized manner in FIG. 2 for an embedded servo disk drive. FIG. 2 is a graphical diagram of an enlarged top view of a prior art embedded servo disk surface showing a data wedge 4 recorded using a constant frequency recording format. A constant frequency signal is recorded in a physical sector 6a of an inner track 6 and in an outer physical sector 8a of an outer track 8 between two servo sample wedges 2. Once servo positioning requirements of a system and the maximum number of flux transitions that may be recorded on the innermost data track of a disk drive are determined, the maximum linear bit density defines the number of data wedges 4 and of servo sample wedges 2 per track.
In a number of embedded servo disk drives known in the art, each data wedge has been used to store only a single sector of data. That is, the fields comprising a sector of data must fit between consecutive servo sample wedges (i.e., the concepts of a "data wedge" and "sector" essentially have been synonymous). In common disk drive types, a data sector typically comprises 512 or 1024 bytes of data, plus header and trailer sections that contain sector synchronization, identification, error correction, and other fields. Accordingly, referring to FIG. 2, even though the outer tracks 8 are longer than the inner tracks 6, and hence are capable of storing more total flux changes at the same FCI rate as the innermost track, such capacity is wasted because the maximum FCI on the innermost track sets a limit on the linear bit density for all outer tracks 8. Thus, constant frequency recording suffers from having a radially decreasing bit density, thereby severely under-utilizing a large portion of the disk surface.
In another prior art drive, data sectors have been split in a fixed and repetitive ratio between adjacent data wedges. However, no known prior art drive has been capable of accommodating a variable or non-integer number of data sectors per data wedge, or arbitrarily splitting data sectors between data wedges.
Another pertinent prior art method is directed to a goal of increasing areal recording density by attempting to achieve what is conventionally termed constant density recording. Ideally, constant density recording would involve the linear bit density of flux changes per unit length remaining constant. Accordingly, the circumferential length of a data sector would remain constant and no longer increase in length with radial displacement outward. In order to accomplish constant linear bit density recording, the recording frequency must increase in proportion to track circumference.
One attempt in the prior art to achieve constant density recording is commonly known as zone bit recording ("ZBR"). In an example of ZBR with embedded servo sectors ("embedded ZBR"), an attempt was made to maximize areal recording density by increasing the number of servo sample wedges 2 from zone to zone. FIG. 3 is a simplified graphical diagram of a top view of a prior art embedded servo disk surface recorded using a zone bit recording format. Concentric zones 10a, 10b, 10c are defined as shown. Zone boundaries 20 may be arbitrarily placed to increase the number of physical sectors per zone 10.
Although embedded ZBR increases the total data storage capacity of the disk surface by maximizing the use of recording area in each zone 10, the servo sample wedges 2 are no longer radially constant for all tracks. This results in a number of disadvantages. Data storage space is lost between zones, because an inter-zone "guard band" (typically two tracks in width) is required at each zone boundary 20 to prevent errors in synchronizing to servo signals on the inner/outer servo sample wedges 2. Because the number of servo sample wedges 2 in outer zones is greater than the number in inner zones, optimization of servo channel rate for servo control efficiency is no longer possible. The servo loop dynamics of the system (the transfer response curve) changes from zone to zone, increasing the complexity of the system. A faster and more expensive processor is required to handle the increased servo channel rate in outer zones. Further, any increase of servo sample wedges 2 above the optimum required for servo control takes away from track space which could be used to increase areal data recording density.
Accordingly, it is desirable to provide a disk formatting technique that provides better storage capacity utilization than present embedded servo disk formats, including embedded ZBR formats, without the complexity of present systems. The present invention provides such a format and technique.